It may take a few seasons of dedication to the weight room and eating right. After seeing Johnson up close with my own eyes I can tell you that he's going to need to add significant bulk if he wants to play Center. He's much thinner and lankier than even KG, so for him to play Center he'll have to do something similar to what DeAndre Jordan has done, and that may take awhile. I think he'll have way more of an effective career as a Power Forward and should just focus on that.
What about JJ at the 4 and KG at the 5? That lineup with Ray/Rondo/Pierce would stretch the defense, wouldn't give up a lot of rebounding power, and it would be great for JJ to get time in with the starters. I could see them doing this mid third quarter with a big lead.
Personally I'm a little wary of KG at the 5 and he's been vocal about not wanting to play the Center position, but if Johnson finds his groove this season I could see them going to that line-up in crunch time (as they did with Posey at the 4 a few seasons ago.)
I've been advocating KG playing some of the C minutes, particularly with JJJ in the fold and with Jeff Green likley returning.
Think about the NBA when KG joined. The big men he'd be banging against at the 5 spot were guys like Shaq, David Robinson, Dikembe, Shawn Kemp, Ewing, Hakeem... I mean, just look at this list of the leading rebounders in KG's rookie year.
http://basketball.realgm.com/src_stats/1996/?sort=rebounds&filter=&value=Now, think about the guys who play the 5 in the current NBA. The only real "bangers" out there these days are Dwight Howard and Z-Bo. We're going to be competing with Chicago; their most important bigs are Noah, Boozer, and Taj Gibson. Miami's got Bosh and they'll probably sign Dalembert. Dallas is Dirk and Tyson Chandler.
There just aren't as many of the bulry, banging-types playing C anymore - the position has changed.
It's a given that we need another guy to play the 5, but it's also still very likely that our four best big men next season will be KG, JO, Jeff Green, and JJJ. It's going to be easier to find a PF than another true C, certainly another true C that'll be any better. Seems like a KG/Green or KG/JJJ frontcourt combination, against a fair number of opponents (not Orlando), is going to be our best bet.
EDIT: KG preaches nothing more than being a team player. Time for him to act on his words and give the team what they need.